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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Charge carrier nonadiabatic dynamics in non-metal doped graphitic carbon nitride

Graphitic carbon nitride (GCN) has attracted significant attention due to its excellent performance in photocatalytic applications. Non-metal doping of GCN has been widely used to improve the efficiency of the material as a photocatalyst. Using a combination of time-domain density functional theory with nonadiabatic molecular dynamics, we study the charge carrier dynamics in oxygen and boron doped GCN systems. The reported simulations provide a detailed time-domain mechanistic description of the charge separation and recombination processes that are of fundamental importance while evaluating the photovoltaic and photocatalytic performance of the material. The appearance of smaller energy gaps due to the presence of dopant states improves the visible light absorption range of the doped systems. At the same time, the nonradiative lifetimes are shortened in the doped systems as compared to the pristine GCN. In the case of boron doped at a carbon (B–C–GCN), the charge recombination time is very long as compared to the other two doped systems owing to the smaller electron–phonon coupling strength between the valence band maximum and the trap state. The results suggest B–C–GCN as the most suitable candidate among three doped systems studied in this work for applications in photocatalysis. Finally, this work sheds light into the influence of dopants on quantum dynamics processes that govern GCN performance and, thus, guides toward building high-performance devices in photocatalysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Identification of the defect dominating high temperature reverse leakage current in vertical GaN power diodes through deep level transient spectroscopy

Deep level defects in wide bandgap semiconductors, whose response times are in the range of power converter switching times, can have a significant effect on converter efficiency. We use Deep Level Transient Spectroscopy (DLTS) to evaluate such defect levels in the n- drift layer of vertical GaN (v-GaN) power diodes with VBD ~ 1500 V. DLTS reveals three energy levels that are at ~0.6 eV (highest density), ~0.27 eV (lowest density) and ~ 45 meV (a dopant level) from the conduction band. Dopant extraction from Capacitance-Voltage measurement test (C-V) at multiple temperatures enables trap density evaluation, and the ~0.6 eV trap has a density of 1.2 × 10 15 cm -3 . Here, the 0.6 eV energy level and its density are similar to a defect that is known to cause current collapse in GaN based surface conducting devices (like HEMTs). Analysis of reverse bias currents over temperature in the v-GaN diodes indicates a predominant role of the same defect in determining reverse leakage current at high temperatures, reducing switching efficiency.

42 ENGINEERING↗
DOE OSTI2022

β -Ga 2 O 3 MESFETs with insulating Mg-doped buffer grown by plasma-assisted molecular beam epitaxy

In this work, we develop in situ Mg doping techniques in plasma-assisted molecular beam epitaxy (PAMBE) of β -Ga 2 O 3 to compensate Si dopants at the substrate epilayer growth interface and eliminate parasitic leakage paths. Both abrupt and uniform Mg doping profiles over a wide range of concentrations were achieved in β -Ga 2 O 3 epilayers grown by PAMBE. Capacitance–voltage characteristics of Si and Mg co-doped samples confirmed the compensating effect of the Mg dopants. Mg delta-doping was then integrated into a β -Ga 2 O 3 metal-semiconductor field effect transistor structure and shown to be effective in eliminating source leakage. In conclusion, the results presented here show that Mg doping is a promising way to engineer insulating buffer layers for β -Ga 2 O 3 lateral devices grown by PAMBE.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Er-doped anatase TiO 2 thin films on LaAlO 3 (001) for quantum interconnects (QuICs)

Rare-earth ions (REIs) doped into solid-state crystal hosts offer an attractive platform for realizing quantum interconnects that can function as quantum memories and quantum repeaters. The 4f valence electrons of REIs are shielded by 5s and 5p electrons and undergo highly coherent transitions even when embedded in host crystals. In particular, Er 3+ has an optical transition in the telecom band that is suitable for low-loss communication. Recently, REIs in thin film systems have gained interest due to potential advantages in providing a flexible host crystal environment, enabling scalable on-chip integration with other quantum devices. Here, we investigate the structural and optical properties of Er-doped anatase TiO 2 thin films on LaAlO 3 (001) substrates. By choosing a system with minimal lattice mismatch and adjusting Er-dopant concentration, we achieve optical inhomogeneous linewidths of 5 GHz at 4.5 K. Furthermore, we show that 9 nm-thick buffer and capping layers can reduce the linewidth by more than 40%, suggesting a pathway to further narrowing linewidths in this system. We also identify that Er 3+ ions mainly incorporate into substitutional Ti 4+ sites with non-polar D 2d symmetry, which makes Er dopants insensitive to the first order to local electric fields from impurities and is desirable for coherence properties of Er 3+ spins.

42 ENGINEERING↗
DOE OSTI2022

Charge-density based evaluation and prediction of stacking fault energies in Ni alloys from DFT and machine learning

A combination of high strength and high ductility has been observed in multi-principal element alloys due to twin formation attributed to low stacking fault energy (SFE). In the pursuit of low SFE alloys, a key bottleneck is the lack of understanding of the composition–SFE correlations that would guide tailoring SFE via alloy composition. Using density functional theory (DFT), we show that dopant radius, which have been postulated as a key descriptor for SFE in dilute alloys, does not fully explain SFE trends across different host metals. Instead, charge density is a much more central descriptor. It allows us to (1) explain contrasting SFE trends in Ni and Cu host metals due to various dopants in dilute concentrations, (2) explain the large SFE variations observed in the literature even within a given alloy composition due to the nearest neighbor environments in “model” concentrated alloys, and (3) develop a machine learning model that can be used to predict SFEs in multi-elemental alloys. Further, this model opens a possibility to use charge density as a descriptor for predicting SFE in alloys.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Acceptor and compensating donor doping of single crystalline SnO (001) films grown by molecular beam epitaxy and its perspectives for optoelectronics and gas-sensing

(La and Ga)-doped tin monoxide [stannous oxide, tin (II) oxide, SnO] thin films were grown by plasma-assisted and suboxide molecular beam epitaxy with dopant concentrations ranging from ≈ 5 × 10 18 to 2 × 10 21 cm -3 . In this concentration range, the incorporation of Ga into SnO was limited by the formation of secondary phases observed at 1.2 × 10 21 cm -3 Ga, while the incorporation of La showed a lower solubility limit. Transport measurements on the doped samples reveal that Ga acts as an acceptor and La as a compensating donor. While Ga doping led to an increase in the hole concentration from 1 × 10 18 -1 × 10 19 cm -3 for unintentionally doped (UID) SnO up to 5 × 10 19 cm -3 , La-concentrations well in excess of the UID acceptor concentration resulted in semi-insulating films without detectable n-type conductivity. Ab initio calculations qualitatively agree with our dopant assignment of Ga and La and further predict In Sn to act as an acceptor as well as Al Sn and B Sn as donors. These results show the possibilities of controlling the hole concentration in p-type SnO, which can be useful for a range of optoelectronic and gas-sensing applications.

36 MATERIALS SCIENCE↗
DOE OSTI2023

First bromine doped cryogenic implosion at the National Ignition Facility

We report on the first experiment dedicated to the study of nuclear reactions on dopants in a cryogenic capsule at the National Ignition Facility (NIF). This was accomplished using bromine doping in the inner layers of the CH ablator of a capsule identical to that used in the NIF shot N140520. The capsule was doped with 3 × 1016 bromine atoms. The doped capsule shot, N170730, resulted in a DT yield that was 2.6 times lower than the undoped equivalent. The Radiochemical Analysis of Gaseous Samples (RAGS) system was used to collect and detect 79Kr atoms resulting from energetic deuteron and proton ion reactions on 79Br. RAGS was also used to detect 13N produced dominantly by knock-on deuteron reactions on the 12C in the ablator. High-energy reaction-in-flight neutrons were detected via the 209Bi(n,4n)206Bi reaction, using bismuth activation foils located 50 cm outside of the target capsule. The robustness of the RAGS signals suggests that the use of nuclear reactions on dopants as diagnostics is quite feasible.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Phosphorus diffusion and deactivation during SiGe oxidation

Dopant profiles near the semiconductor–oxide interface are critical for microelectronic device performance. As the incorporation of Si 1−x Ge x into transistors continues to increase, it is necessary to understand the behavior of dopants in Si 1−x Ge x . In this paper, the diffusion and electrical activation of phosphorus within a strained, single-crystal Si 0.7 Ge 0.3 layer on Si during oxidation are reported. Both layers were uniformly doped, in situ, with an average phosphorus concentration of 4 × 10 19 atoms/cm 3 . After high-temperature oxidation, secondary ion mass spectrometry measurements revealed that the bulk of the phosphorus diffuses out of only the SiGe layer and segregates at the oxidizing SiGe–SiO 2 interface. Hall effect measurements corroborate the observed phosphorus loss and show that the phosphorus diffusing to the oxidizing interface is electrically inactive. Through density functional theory (DFT) calculations, it is shown that phosphorus interstitials prefer sites near the SiGe–SiO 2 interface. Finally, based on a combination of experimental data and DFT calculations, we propose that the phosphorus atoms are displaced from their lattice sites by Ge interstitials that are generated during SiGe oxidation. The phosphorus atoms then migrate toward the SiGe–SiO 2 interface through a novel mechanism of hopping between Ge sites as P–Ge split interstitials. Once they reach the interface, they are electrically inactive, potentially in the form of interstitial clusters or as part of the reconstructed interface or oxide.

Thornton, Chappel S.↗
DOE OSTI2023

Electronic structure of boron and aluminum δ -doped layers in silicon

Recent work on atomic-precision dopant incorporation technologies has led to the creation of both boron and aluminum δ-doped layers in silicon with densities above the solid solubility limit. For this study, we use density functional theory to predict the band structure and effective mass values of such δ layers, first modeling them as ordered supercells. Structural relaxation is found to have a significant impact on the impurity band energies and effective masses of the boron layers, but not the aluminum layers. However, disorder in the δ layers is found to lead to a significant flattening of the bands in both cases. We calculate the local density of states and doping potential for these δ-doped layers, demonstrating that their influence is highly localized with spatial extents at most 4 nm. We conclude that acceptor δ-doped layers exhibit different electronic structure features dependent on both the dopant atom and spatial ordering. This suggests prospects for controlling the electronic properties of these layers if the local details of the incorporation chemistry can be fine-tuned.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Acoustic and optical phonon frequencies and acoustic phonon velocities in Si-doped AlN thin films

We report the results of the study of the acoustic and optical phonons in Si-doped AlN thin films grown by metal–organic chemical vapor deposition on sapphire substrates. The Brillouin–Mandelstam and Raman light scattering spectroscopies were used to measure the acoustic and optical phonon frequencies close to the Brillouin zone center. The optical phonon frequencies reveal non-monotonic changes, reflective of the variations in the thin film strain and dislocation densities with the addition of Si dopant atoms. The acoustic phonon velocity decreases monotonically with increasing Si dopant concentration, reducing by ∼300 m/s at the doping level of 3 × 1019 cm−3. The knowledge of the acoustic phonon velocities can be used for the optimization of the ultra-wide bandgap semiconductor heterostructures and for minimizing the thermal boundary resistance of high-power devices.

Physics↗
DOE OSTI2024

Study of phase decoherence in GeSn (8%) through measurements of the weak antilocalization effect

Alloying germanium with tin offers a means to modulate germanium's electronic structure, enabling a greater degree of control over quantum properties such as the retention of the phase or spin of the electron wave. However, the extent to which the presence of high dopant concentrations in GeSn alters these quantum behaviors is poorly understood. Here, we investigate the role of dopant concentrations on phase coherence through measurements of the weak antilocalization (WAL) effect at temperatures between 30 mK and 10 K in p-GeSn (8%) thin films, which were doped to a series of carrier densities on the order of 10 12 cm –2 . Phase coherence and spin–orbit lengths were extracted from the magnetoconductivities using the 2D Hikami–Larkin–Nagaoka model. Phase coherence lengths peaked at 577, 593, and 737 nm for the low-, mid-, and high-density samples, while upper limits on the spin–orbit lengths of less than 25 nm were relatively independent of carrier density and temperature. The phase coherence lengths increased as the temperature decreased but changed only minimally with carrier density, contrary to common models of temperature-dependent inelastic scattering. Saturation of the phase coherence lengths occurred below 600 mK. Based on these findings, intrinsically generated inelastic scattering mechanisms such as two-level systems or impurity band scattering likely contribute to phase decoherence in these alloys. Our results provide insight into the inelastic scattering mechanisms of GeSn, while suggesting a need for further investigation into phase decoherence mechanisms in doped group-IV alloys.

42 ENGINEERING↗
DOE OSTI2025

Arsenic activation and compensation in single crystal CdTe bilayers

In state-of-the art polycrystalline CdTe photovoltaics, group-V dopant activation is about 2%. Low activation can create electronic defects and lead to recombination and band tail losses. To develop methods to overcome this limitation, dopant activation was systematically investigated using molecular beam epitaxy (MBE) grown single crystal bilayers of As-doped CdTe on undoped CdTe. Results suggest multiple paths for improved As-activation in polycrystalline CdTe-based devices. It was found that the carrier concentration in this MBE material saturated at ∼3 × 1016 cm−3, with high levels (>50%) of As-activation possible. High activation could be achieved with a post-growth activation temperature of ∼450 °C, when the initial doping level was below the saturation level. However, at typical polycrystalline As incorporation levels (>5 × 1016 cm−3), the excess As is inactive or compensating, requiring elevated temperatures (500–600 °C) to achieve high activation. Oxygen in the annealing ambient was detrimental, while the effect of CdCl2 in the ambient is more case-dependent. A 575 °C activation anneal was combined with a 450 °C CdCl2 treatment to better understand the implications for polycrystalline CdTe. Interestingly, on highly doped samples, processes ending with a high temperature step displayed high activation, while those ending at 450 °C significantly reduced the carrier concentration (with or without CdCl2 in the ambient). Low activation can be restored with another high temperature anneal, allowing reproducible toggling between high and low activation based on the final temperature. Photoluminescence revealed the presence of donor–acceptor pairs in the low activation state that appear to be associated with a compensating defect.

14 SOLAR ENERGY↗
DOE OSTI2025

Epitaxial strain tuning of Er 3+ in ferroelectric thin films

Er 3+ color centers are promising candidates for quantum science and technology due to their long electron and nuclear spin coherence times, as well as their desirable emission wavelength. By selecting host materials with suitable, controllable properties, we introduce new parameters that can be used to tailor the Er 3+ emission spectrum. PbTiO 3 is a well-studied ferroelectric material with known methods of engineering different domain configurations through epitaxial strain. By distorting the structure of Er 3+ -doped PbTiO 3 thin films, we can manipulate the crystal fields around the Er 3+ dopant. This is resolved through changes in the Er 3+ resonant fluorescence spectra, tying the optical properties of the defect directly to the domain configurations of the ferroelectic matrix. Additionally, we are able to resolve a second set of peaks for films with in-plane ferroelectric polarization. We hypothesize these results to be due to either the Er 3+ substituting different sites of the PbTiO 3 crystal, differences in charges between the Er 3+ dopant and the original substituent ion, or selection rules. Systematically studying the relationship between the Er 3+ emission and the epitaxial strain of the ferroelectric matrix lays the pathway for future optical studies of spin manipulation by altering ferroelectric order parameters.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Electric-field-induced crossover of polarization reversal mechanisms in Al 1–x Sc x N ferroelectrics

Scandium-doped aluminum nitride, Al 1–x Sc x N, represents a new class of displacive ferroelectric materials with high polarization and sharp hysteresis along with high-temperature resilience, facile synthesizability and compatibility with standard CMOS fabrication techniques. The fundamental physics behind the transformation of unswitchable piezoelectric AlN into switchable Al–Sc–N ferroelectrics depends upon important atomic properties such as local structure, dopant distributions and the presence of competing mechanism of polarization switching in the presence of an applied electric-field that have not been understood. We computationally synthesize Al 1–x Sc x N to quantify the inhomogeneity of Sc distribution and phase segregation, and characterize its crystal and electronic structure as a function of Sc-doping. Nudged elastic band calculations of the potential energy surface and quantum molecular dynamics simulations of direct electric-field-driven ferroelectric switching reveal a crossover between two polarization reversal mechanisms—inhomogeneous nucleation-and-growth mechanism originating near Sc-rich regions in the limit of low applied fields and nucleation-limited-switching in the high-field regime. Finally, understanding polarization reversal pathways for these two mechanisms as well as the role of local Sc concentration on activation barriers provides design rules to identify other combinations of dopant elements, such as Zr, Mg etc. to synthesize superior AlN-based ferroelectric materials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

P-type conductivity in Sn-doped Sb 2 Se 3

Abstract Antimony selenide (Sb 2 Se 3 ) is a promising absorber material for thin-film photovoltaics. However, certain areas of fundamental understanding of this material remain incomplete and this presents a barrier to further efficiency gains. In particular, recent studies have highlighted the role of majority carrier type and extrinsic doping in drastically changing the performance of high efficiency devices (Hobson et al 2020 Chem. Mater. 32 2621–30). Herein, Sn-doped Sb 2 Se 3 bulk crystals are shown to exhibit p-type conductivity using Hall effect and hot-probe measurements. The measured conductivities are higher than those achieved through native defects alone, but with a carrier density (up to 7.4 × 10 14 cm −3 ) several orders of magnitude smaller than the quantity of Sn included in the source material. Additionally, a combination of ultraviolet, x-ray and hard x-ray photoemission spectroscopies are employed to obtain a non-destructive depth profile of the valence band maximum, confirming p-type conductivity and indicating a majority carrier type inversion layer at the surface. Finally, these results are supported by density functional theory calculations of the defect formation energies in Sn-doped Sb 2 Se 3 , showing a possible limit on the carrier concentration achievable with Sn as a dopant. This study sheds light on the effectiveness of Sn as a p-type dopant in Sb 2 Se 3 and highlights avenues for further optimisation of doped Sb 2 Se 3 for solar energy devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Using Fourier-Plane Imaging Microscopy for Determining Transition-Dipole-Moment Orientations in Organic Light-Emitting Devices

Here, we use Fourier-plane imaging microscopy (FIM) to determine the transition-dipole-moment orientation in doped organic emissive thin films. The use of FIM enables precise, sensitive, and rapid measurement of dipole orientation in the emission layer of an organic light-emitting device (OLED). An optical model of a stratified birefringent multilayer is introduced for interpreting results obtained by FIM. Using the model, we determine the average orientation of transition-dipole-moment vectors of three phosphorescent dopant emitters. The dipole alignment measured by FIM quantitatively explains the difference in OLED efficiencies using these archetype dopant molecules. FIM provides a nondestructive tool to measure and ultimately improve the outcoupling efficiency of OLEDs and other light-emitting devices.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2024

Possible coexistence of magnetism and paramagnetic singularity in lightly Fe-doped WTe 2

Topological semimetals possess nodal or nodal-line phases where conduction and valence bands touch at points or lines in momentum space, respectively. Such band touching is symmetry protected and gives rise to exotic and interesting electronic properties. Coupling topological order with magnetism provides a platform for exploring time-reversal (TR) symmetry breaking topological physics, such as axion electrodynamics, inverse spin-galvanic effect, and the quantized anomalous Hall effect. The Weyl semimetal (WSM) requires breaking either TR symmetry or lattice inversion symmetry (I). By doping inversion-symmetry-broken WSM with magnetic dopants, one can expect to create a WSM with both symmetries breaking simultaneously. Here, structural, electrical, and magnetic properties of Fe x ⁢W 1–x Te 2 (x = 0 and 0.011) are reported. It is revealed that, with a small Fe doping concentration (x = 0.011), a ferromagnetism is induced at low temperature (<10 K). Scanning tunneling microscopy and spectroscopy measurements in Fe 0.011 ⁢W 0.989 ⁢Te 2 further reveal only substitution and no intercalated dopants being observed. The probabilities of the Fe substitutions at the two nonequivalent W sites are quantified with equal probability. The dl/dV point spectra indicates that the Fe substitution in WTe 2 manifests itself as electron doping regardless of doping sites. The results clearly reveal the possible coexistence of magnetism and Weyl points in the lightly Fe doped WTe2 at low temperature. Furthermore, this provides an ideal system for further study on the interplay between the topological Weyl points and the TR symmetry breaking.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Large inverse Faraday effect for Rydberg states of free atoms and isolated donors in semiconductors

We report on the induction of magnetization in Rydberg systems by means of the inverse Faraday effect and propose the appearance of the effect in two such systems: Rydberg atoms proper and shallow dopants in semiconductors. Rydberg atoms are characterized by a large orbital radius. This large radius gives such excited states a large angular moment, which when driven with circularly polarized light translates to a large effective magnetic field $\mathscr{B}$ eff . We calculate this effect to generate effective magnetic fields of O⁡(1⁢ µ⁢T)×($\frac{ω}{1THz}$) -1 ⁢ ($\frac{\mathscr{I}}{10Wcm^{-2}}$)⁢n 4 in the Rydberg states of atoms such as Rb and Cs for off-resonant photon beams with frequency ω and intensity $\mathscr{I}$ expressed in units of the denominators and n the principal quantum number. Additionally, terahertz spectroscopy of phosphorus-doped silicon reveals a large cross section for excitation of shallow dopants to Rydberg-like states, which even for small n have the potential to be driven similarly with circularly polarized light to produce an even larger magnetization. Our theoretical calculations estimate $\mathscr{B}$ eff as O⁡(10 2 T) for Si:P with a beam intensity of 10 8 Wcm -2 .

36 MATERIALS SCIENCE↗